The present invention relates to a transmission control device.
For example, an automatic transmission for an automotive vehicle and the like is equipped with a hydraulic transmission device, such as a torque converter, provided on a driving-power transmitting path between an engine and driving wheels, in which there is provided a lockup clutch directly coupling an input element such as a pump with an output element such as a turbine. The lockup clutch is controlled according to a vehicle driving condition based on an engine load, a vehicle speed and so on such that, for instance, it is in a lockup state in a low engine-load and high vehicle-speed region, it is in a converter state in a high engine-load and low vehicle-speed region, and it is in a slip state in a low engine-load and middle vehicle-speed region. This control could improve gas mileage in the lockup state by a full (direct) coupling of the input element and the output element of the hydraulic transmission device, engine torque multiplication in the converter state by a full release of them, and both of gas mileage and drivability (i.e., absorption of shock or vibration due to a torque change) in the slip state by a half (partial) coupling of them. However, this kind of lockup clutch has a relatively large diameter, a piston chamber with a large volume, and a heavy piston, thereby providing a large vibration of its piston's face. Accordingly, this may cause disadvantages that its responsiveness, controllability and thereby controlling accuracy are inferior.
Meanwhile, it is known that there is provided other frictional coupling elements provided on the driving-power transmitting path between the engine and the driving wheel, which are controlled in a certain slip state respectively. Herein, the frictional coupling elements include, for example, the followings: clutches for achieving gear shift, such as a forward clutch, used for selecting a plurality of transmission gears by changing a driving-power transmitting path of a planetary gear mechanism; a starter clutch used for decreasing a rotation load of the engine by connecting or disconnecting the driving-power transmission between the engine and the fluid transmission device or between the fluid transmission device and the transmission; an forward-backward switching clutch used mainly along with a continuously variable transmission and for changing a rotational direction of an output of the transmission; a low-mode clutch and a high-mode clutch used along with the continuously variable transmission to enable a geared neutral and for changing the driving-power transmitting path to a low mode with a large gear ratio and a high mode with a small gear ratio respectively; and so on. Generally, this kind of frictional coupling elements have a relatively small diameter, a piston chamber with a small volume, and a light-weight piston, thereby providing a small vibration of its piston's face, compared with the lockup clutch of the hydraulic transmission device. Accordingly, they have advantages that its responsiveness, controllability and thereby control accuracy are superior.
For example, Japanese Patent Laid-Open Publication No. 2000-304125 discloses a control, referred to as a neutral control, that a frictional coupling element (forward clutch) for selecting a forward first-gear is controlled to be in a certain slip state during an idling stop with a D (drive) range in order to improve gas mileage and suppress an idling vibration. This control can reduce the load torque added to the engine by a reverse function of a creeping phenomena, thereby improving gas mileage and suppressing the idling vibration. Further, the U.S. Pat. No. 5,400,678 discloses a control that a frictional coupling element which is the weakest one, in other words, which requires the highest operating pressure, is controlled so as to be slightly slipped (to a certain extent that a durability of the frictional coupling element may not deteriorate: for example, 10 rpm) in order to achieve the best control of a line pressure and the best efficiency of transmission.
Herein, in the invention disclosed in the above-described Japanese patent publication, the slip control of the forward clutch is executed only when a vehicle stops, and the forward clutch is controlled in its lockup state when or after the vehicle starts. In the invention disclosed in the above-described US patent, the slip control of the frictional coupling element is executed in order to determine a certain line pressure, being increased gradually from the slip state, which can prevent the frictional coupling element from slipping. Namely, in these conventional ways, these slip controls of the frictional coupling element have been just executed in certain limited conditions and for certain limited purposes. An associated control between the slip control of the frictional coupling element and the slip control of the lockup clutch has not been executed.
Of course, it is known that the slip control of the frictional coupling element is executed for achieving the gear shift and the lockup clutch is made to slip for a short period of time in order to absorb a shock during gear shift. However, since the conventional controls did not perform so as to associate the slip control of the frictional coupling element with the slip control of the lockup clutch, they were unable to improve both of gas mileage and drivability (shock suppression and so on) properly.